From 597e0d95fe597f7cc4eb10d5e4a218f134f824d8 Mon Sep 17 00:00:00 2001
From: Luke <luke.wilde@live.co.uk>
Date: Wed, 9 Jun 2021 04:43:13 +0100
Subject: [PATCH] LibJS: Only set element in array literal to an empty value if
 it's null

This avoids an unnecessary empty load that immediately gets overridden.

For example, `[1,,]` would appear as:
[   0] EnterScope
[  10] LoadImmediate value:<empty>
[  28] LoadImmediate value:1
[  40] Store dst:$1
[  48] LoadImmediate value:<empty>
[  60] Store dst:$2
[  68] NewArray, elements:[$1,$2]

But now appears as:
[   0] EnterScope
[  10] LoadImmediate value:1
[  28] Store dst:$1
[  30] LoadImmediate value:<empty>
[  48] Store dst:$2
[  50] NewArray, elements:[$1,$2]
---
 Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
index c72fd47110..732915e4ca 100644
--- a/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
+++ b/Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp
@@ -462,7 +462,6 @@ void ArrayExpression::generate_bytecode(Bytecode::Generator& generator) const
 {
     Vector<Bytecode::Register> element_regs;
     for (auto& element : m_elements) {
-        generator.emit<Bytecode::Op::LoadImmediate>(Value {});
         if (element) {
             element->generate_bytecode(generator);
 
@@ -470,6 +469,8 @@ void ArrayExpression::generate_bytecode(Bytecode::Generator& generator) const
                 TODO();
                 continue;
             }
+        } else {
+            generator.emit<Bytecode::Op::LoadImmediate>(Value {});
         }
         auto element_reg = generator.allocate_register();
         generator.emit<Bytecode::Op::Store>(element_reg);